1. Field of the Invention
The present invention is in the field of curing thick, non-uniform rubber articles, and more particularly in the field of curing tires such as truck tires.
2. Description of the Related Art
Rubber articles, such as pneumatic tires, for years have been vulcanized or cured in a press wherein heat is applied externally by the tire mold and internally by a curing bladder for a certain length of time to effect vulcanization of the tire. These presses, which are well known in the art, generally employ separable mold halves or parts (including segmented mold parts) with shaping and curing mechanisms, and utilize bladders into which shaping, heating and cooling fluids or media are introduced for curing the articles. The aforesaid curing presses typically are controlled by a mechanical timer or a programmable logic controller (PLC) which cycles the presses through various steps during which the article is shaped, heated and in some processes cooled prior to unloading from the press. During the curing process the article is subjected to high pressure and high temperature for a preset period of time which is set to provide sufficient cure of the thickest part of the article. The cure process usually continues to completion outside the press.
Rubber chemists are faced with the problem of predicting the time period within which each part of the rubber article will be satisfactorily cured and, once such a time period is established, the article is heated for that period. This is a relatively straight-forward analysis for a rubber article that is relatively thin and has uniform geometry and similar composition throughout. It is a much more difficult analysis when this is not the situation such as with a complex article such as a tire, and particularly with large tires such as truck tires, off-the-road tires, farm, aircraft and earthmover tires. The extent of cure in these types of tires is affected not only by the variance in geometry from part to part in the tire but also by the composition change and laminate structure as well. While the time control method has been used to cure millions of tires, because of the varying composition and geometry in the tire, some parts of the tire tend to be more cured than other parts. By setting the time period to cure the most difficult part to cure, over-cure of some parts occurs, and production time on the vulcanizing machinery is wasted and production efficiency is reduced.
Various designs for curing presses and various curing methods have been proposed to provide a more uniform cure to thick rubber articles. Some methods use differing materials for mold construction, insulating materials, differing compositions for parts of the tire, multiple curing zones so heat can be applied for a longer time, or methods for directing more heat to the thickest or most complex part of the rubber article. Patents which show various curing methods and apparatus are as follows: U.S. Pat. Nos.: 3,718,721; 3,819,915; 4,371,483; 4,044,600; 4,861,253; 5,055,245; 5,481,319; 6,413,068; and 6,478,991; EPO Patent Applications 0,485,127A1; 0,578,105A2 and 1,172,198A2; and Japanese Patents JP62037107; JP 0,7,195,370 and JP 2002,172,622A. However, none of the above methods and apparatus has proven entirely satisfactory, and time control remains the typical method of curing non-uniform thick rubber articles. Thus, the tire industry is faced with an issue of producing a uniformly cured tire in a faster time period.
The invention is directed to an improved method of curing a non-uniform, thick rubber article such as a truck tire or a tread for a tire. The method uses heat transfer elements which are specifically chosen and specifically placed in a mold to transfer heat into the article at the cure-limiting parts of the article. This results in small apertures in the articles where the heat transfer elements protrude into the article. Since these apertures are small, they do not change the relative function and performance of the article. The apertures are basically seen as pin holes or mini-sipes.
Siping is well known in the tire art to create functional advantages such as traction, water evacuation, etc. U.S. Pat. Nos. 6,196,288; 4,298,046; 5,289,862 and 5,896,905 disclose various effects of siping with tires. The distinction between the use of siping in the prior art and the use of the mini-sipes of this invention is that the siping in the prior art is directed to causing or creating a functional change in the tire where the mini-sipes of the invention are chosen to effect an improved cure without changing the relative functional performance of the tire or tread being cured.
As for the pin holes, the same distinction applies between the prior art and the invention. Pin holes in the tread are known in the tire prior art to create functional changes in wear, traction, water evacuation, performance, etc. (See U.S. Pat. Nos.: 2,504,090; 3,409,064; 3,637,001; 3,645,313; 3,749,145; 3,998,256; 4,266,592; 4,305,445; 4,723,584; 5,027,876 and 6,374,886; U.S. Patent Publication Nos. 2003/0089439 and 2002/0100527; and Great Britain Patent No. 546,975. Other patents show lateral pin holes introduced into the side of the tread of a tire to create a functional change in the tire performance. (See U.S. Pat. Nos.: 1,408,100; 1,509,259; 1,733,064; 1,778,071; 1,877,988; 3,848,651; and 4,979,549; Japanese Patent Publication No. P2002-307442A and Japanese Application No. 3-355071).